Roller bearings for high ratio geared turbofan engine

ABSTRACT

A gear system for a geared turbofan engine is disclosed. The gear system includes a sun gear driven by a low spool shaft. The sun gear defines a sun gear diameter. A rotating carrier drives a fan. The carrier defines an outer carrier diameter and an inner carrier diameter. A non-rotating ring gear is also included. The ring gear defines a ring gear diameter and the ring gear diameter is smaller than the outer carrier diameter. A set of planet gears are mounted on corresponding rolling element bearing assemblies. Each roller element bearing assembly is supported within the carrier within a space defined between the carrier outer diameter and the carrier inner diameter. Each of the sun gear, ring gear and planet gears are substantially centered along a gearbox centerline transverse to an engine longitudinal axis and the gear system provides a speed reduction ratio between an input to the sun gear and an output from the carrier between 3:1 and 5:1. A method of creating a gear system for a geared turbofan engine and a geared turbofan system are also disclosed.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application Ser. No. 62/183,384 filed Jun. 23, 2015.

BACKGROUND

A gas turbine engine typically includes a fan section, a compressor section, a combustor section and a turbine section. Air entering the compressor section is compressed and delivered into the combustion section where it is mixed with fuel and ignited to generate a high-energy exhaust gas flow. The high-energy exhaust gas flow expands through the turbine section to drive the compressor and the fan section. The compressor section typically includes low and high pressure compressors, and the turbine section includes low and high pressure turbines.

A speed reduction device such as an epicyclical gear assembly may be utilized to drive the fan section such that the fan section may rotate at a speed different than the turbine section so as to increase the overall propulsive efficiency of the engine. In such engine architectures, a shaft driven by one of the turbine sections provides an input to the epicyclical gear assembly that drives the fan section at a reduced speed such that both the turbine section and the fan section can rotate at closer to optimal speeds.

Although geared architectures have improved propulsive efficiency, turbine engine manufacturers continue to seek further improvements to engine performance including improvements to thermal, transfer and propulsive efficiencies.

SUMMARY

In a featured embodiment, a gear system for a geared turbofan engine in which the gear system includes a sun gear driven by a low spool shaft. The sun gear defines a sun gear diameter. A rotating carrier drives a fan. The carrier defines an outer carrier diameter and an inner carrier diameter. A non-rotating ring gear is also included. The ring gear defines a ring gear diameter and the ring gear diameter is smaller than the outer carrier diameter. A set of planet gears are mounted on corresponding rolling element bearing assemblies. Each roller element bearing assembly is supported within the carrier within a space defined between the carrier outer diameter and the carrier inner diameter. Each of the sun gear, ring gear and planet gears are substantially centered along a gearbox centerline transverse to an engine longitudinal axis and the gear system provides a speed reduction ratio between an input to the sun gear and an output from the carrier between 3:1 and 5:1.

In another embodiment according to the previous embodiment, the set of planet gears includes between 3 and 5 planet gears, inclusive.

In another embodiment according to any of the previous embodiments, an outer periphery of the roller element bearings extends past the ring gear diameter.

In another embodiment according to any of the previous embodiments, the carrier includes a forward portion attached to an aft portion with the set of planet gears supported between the forward portion and the aft portion.

In another embodiment according to any of the previous embodiments, the ring gear diameter includes a pitch diameter.

In another embodiment according to any of the previous embodiments, the sun gear diameter is greater than carrier inner diameter.

In another embodiment according to any of the previous embodiments, the rolling element bearing assemblies include one of a roller, ball, tapered roller, and spherical bearing.

In another embodiment according to any of the previous embodiments, the rolling element bearing assemblies include multiple rows or rolling element bearings.

In another featured embodiment, a method of creating a gear system for a geared turbofan engine, the method includes configuring a carrier to include an outer diameter and an inner diameter about a gearbox centerline transverse to an engine longitudinal axis. A sun gear is configured to be supported on an input shaft. The input shaft includes a diameter less than the carrier inner diameter and the sun gear includes a sun gear pitch diameter greater than the carrier inner diameter. A ring gear is configured to include a ring gear pitch diameter. A set of planet gears is configured for assembly into the carrier. Each of the sun gear, the ring gear and the set of planet gears are configured to be centered along the gearbox centerline. At least one rolling element bearing assembly is configured to include a bearing outer diameter that fits within a space between the carrier outer diameter and the carrier inner diameter. A bearing life of the at least one rolling element bearing assembly is confirmed based upon the bearing outer diameter being within a desired range. The set of planet gears is configured to include between 3 and 5 gears, inclusive, responsive to the size of the bearing outer diameter.

In another embodiment according to the previous embodiment, includes configuring the ring gear pitch diameter to be less than the carrier outer diameter.

In another embodiment according to any of the previous embodiments, includes selecting the number of planet gears within the set of planet gears to provide the greatest number of planet gears with a rolling element bearing assembly within the desired range.

In another embodiment according to any of the previous embodiments, includes configuring the carrier to include a forward portion attached to an aft portion and supporting the set of planet gears between the forward portion and the aft portion.

In another embodiment according to any of the previous embodiments, includes supporting a rolling element bearing assembly within each of the forward portion and the aft portion.

In another embodiment according to any of the previous embodiments, includes configuring the rolling element bearing assemblies to include at least one of a roller, ball, tapered roller, and spherical bearing.

In a featured embodiment, a geared turbofan engine includes a fan section rotatable about an engine longitudinal axis, a turbine section, and a gear system driven by the turbine section. The gear system includes a sun gear driven by a low spool shaft. The sun gear defines a sun gear diameter. A rotating carrier drives the fan section. The carrier defines an outer carrier diameter and an inner carrier diameter. A non-rotating ring gear is also included. The ring gear defines a ring gear diameter and the ring gear diameter is smaller than the carrier outer diameter. A set of planet gears are mounted on corresponding rolling element bearing assemblies. Each of the roller element bearing assemblies are being supported within the carrier within a space defined between the carrier outer diameter and the carrier inner diameter. Each of the sun gear, ring gear and planet gears are substantially centered along a gearbox centerline transverse to an engine longitudinal axis and the gear system provides a speed reduction ratio between an input to the sun gear and an output from the carrier between 3:1 and 5:1.

In another embodiment according to the previous embodiment, the set of planet gears includes between 3 and 5 planet gears, inclusive.

In another embodiment according to any of the previous embodiments, an outer periphery of each of the roller element bearing assemblies extends past the ring gear diameter.

In another embodiment according to any of the previous embodiments, the carrier includes a forward portion attached to an aft portion with the set of planet gears supported between the forward portion and the aft portion.

In another embodiment according to any of the previous embodiments, the rolling element bearing assemblies include one of a roller, ball, tapered roller, and spherical bearing.

Although the different examples have the specific components shown in the illustrations, embodiments of this disclosure are not limited to those particular combinations. It is possible to use some of the components or features from one of the examples in combination with features or components from another one of the examples.

These and other features disclosed herein can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an embodiment of a geared turbofan engine.

FIG. 2 schematically shows an example gear system for a geared turbofan engine.

FIG. 3 is a cross-section of an example gear system embodiment.

FIG. 4 is the cross-section of the example gear system embodiment illustrating example features for sizing rolling element bearing assemblies.

FIG. 5 is a schematic view of the gear system further illustrating features for sizing rolling element bearing assemblies.

FIG. 6 is chart illustrating an example method for configuring a gear system including rolling element bearing assemblies.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates an example geared turbofan engine 20 that includes a fan section 22, a compressor section 24, a combustor section 26 and a turbine section 28. Alternative engines might include an augmenter section (not shown) among other systems or features. The fan section 22 drives air along a bypass flow path B while the compressor section 24 draws air in along a core flow path C where air is compressed and communicated to a combustor section 26. In the combustor section 26, air is mixed with fuel and ignited to generate a high energy exhaust gas stream that expands through the turbine section 28 where energy is extracted and utilized to drive the fan section 22 and the compressor section 24.

Although the disclosed non-limiting embodiment depicts a two-spool turbofan gas turbine engine, it should be understood that the concepts described herein are not limited to use with two-spool turbofans as the teachings may be applied to other types of turbine engines; for example a turbine engine including a three-spool architecture in which three spools concentrically rotate about a common axis and where a low spool enables a low pressure turbine to drive a fan via a gearbox, an intermediate spool that enables an intermediate pressure turbine to drive a first compressor of the compressor section, and a high spool that enables a high pressure turbine to drive a high pressure compressor of the compressor section.

The example engine 20 generally includes a low speed spool 30 and a high speed spool 32 mounted for rotation about an engine central longitudinal axis A relative to an engine static structure 36 via several bearing systems 38. It should be understood that various bearing systems 38 at various locations may alternatively or additionally be provided.

The low speed spool 30 generally includes an inner shaft 40 that connects a fan 42 and a low pressure (or first) compressor section 44 to a low pressure (or first) turbine section 46. The inner shaft 40 drives the fan 42 through a speed change device, such as a gear system 48, to drive the fan 42 at a lower speed than the low speed spool 30. The high-speed spool 32 includes an outer shaft 50 that interconnects a high pressure (or second) compressor section 52 and a high pressure (or second) turbine section 54. The inner shaft 40 and the outer shaft 50 are concentric and rotate via the bearing systems 38 about the engine central longitudinal axis A.

A combustor 56 is arranged between the high pressure compressor 52 and the high pressure turbine 54. In one example, the high pressure turbine 54 includes at least two stages to provide a double stage high pressure turbine 54. In another example, the high pressure turbine 54 includes only a single stage. As used herein, a “high pressure” compressor or turbine experiences a higher pressure than a corresponding “low pressure” compressor or turbine.

The example low pressure turbine 46 has a pressure ratio that is greater than about 5. The pressure ratio of the example low pressure turbine 46 is measured prior to an inlet of the low pressure turbine 46 as related to the pressure measured at the outlet of the low pressure turbine 46 prior to an exhaust nozzle.

A mid-turbine frame 58 of the engine static structure 36 is arranged generally between the high pressure turbine 54 and the low pressure turbine 46. The mid-turbine frame 58 further supports bearing systems 38 in the turbine section 28 as well as setting airflow entering the low pressure turbine 46.

Airflow through the core airflow path C is compressed by the low pressure compressor 44 then by the high pressure compressor 52 mixed with fuel and ignited in the combustor 56 to produce high energy exhaust gases that are then expanded through the high pressure turbine 54 and low pressure turbine 46. The mid-turbine frame 58 includes vanes 60, which are in the core airflow path and function as an inlet guide vane for the low pressure turbine 46. Utilizing the vane 60 of the mid-turbine frame 58 as the inlet guide vane for low pressure turbine 46 decreases the length of the low pressure turbine 46 without increasing the axial length of the mid-turbine frame 58. Reducing or eliminating the number of stages in the low pressure turbine 46 shortens the axial length of the turbine section 28. Thus, the compactness of the gas turbine engine 20 is increased and a higher power density may be achieved.

The disclosed gas turbine engine 20 in one example is a high-bypass geared aircraft engine. In a further example, the gas turbine engine 20 includes a bypass ratio greater than about six (6), with an example embodiment being greater than about ten (10). The example gear system 48 is an epicyclical gear train, such as a planetary gear system, star gear system or other known gear system, with a gear reduction ratio of greater than about 2.3. In another example embodiment the gear system provides a speed reduction ratio greater than about 3:1. In yet another example embodiment the gear system 48 provides a speed reduction ratio between about 3:1 and about 5:1.

In one disclosed embodiment, the geared turbofan engine 20 includes a bypass ratio greater than about ten (10:1) and the fan diameter is significantly larger than an outer diameter of the low pressure compressor 44. It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared turbofan engine including a gear system and that the present disclosure is applicable to other gas turbine engines.

A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The fan section 22 of the engine 20 is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet. The flight condition of 0.8 Mach and 35,000 ft., with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (‘TSFC’)”—is the industry standard parameter of pound-mass (lbm) of fuel per hour being burned divided by pound-force (lbf) of thrust the engine produces at that minimum point.

“Low fan pressure ratio” is the pressure ratio across the fan blades alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.50. In another non-limiting embodiment the low fan pressure ratio is less than about 1.45.

“Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram ° R)/(518.7° R)]^(0.5). The “Low corrected fan tip speed”, as disclosed herein according to one non-limiting embodiment, is less than about 1150 ft/second. The fan tip speed is understood by those skilled in the art to be measured at engine operating conditions associated with takeoff of an aircraft.

The example geared turbofan engine 20 includes the fan 42 that comprises in one non-limiting embodiment less than about 26 fan blades. In another non-limiting embodiment, the fan section 22 includes less than about 20 fan blades. Moreover, in one disclosed embodiment the low pressure turbine 46 includes no more than about 6 turbine rotors schematically indicated at 34. In another non-limiting example embodiment the low pressure turbine 46 includes about 3 turbine rotors. A ratio between the number of fan blades 42 and the number of low pressure turbine rotors is between about 3.3 and about 8.6. The example low pressure turbine 46 provides the driving power to rotate the fan section 22 and therefore the relationship between the number of turbine rotors 34 in the low pressure turbine 46 and the number of blades 42 in the fan section 22 disclose an example geared turbofan engine 20 with increased power transfer efficiency.

Referring to FIGS. 2 and 3 with continued reference to FIG. 1, an example gear system 48 includes a sun gear 62 that is driven about the engine axis A by an input shaft 64. The input shaft 64 is driven by the inner shaft 40 that, in this example, is driven by the low pressure turbine 46. The sun gear 62 drives a plurality of planet gears 102 supported by a carrier 70. The planet gears 102 rotate within a ring gear 82 that is attached by fixed support 86 to the static structure 36 of the engine 20 such that the ring gear 82 does not rotate.

The example carrier 70 rotates about the engine axis A and drives a fan shaft 88 that drives the fan section 22. Each of the sun gear 62, planet gears 102 and the ring gear 82 are centered about a gear system centerline 80. The centerline 80 is disposed transverse to the engine axis A and defines a single stage gear system 48.

The carrier 70 includes a forward portion 76 attached to an aft portion 78 by fastener assemblies 118 (only one shown in FIG. 3). The aft portion 78 of the carrier 70 includes an inner diameter 74 disposed about the engine axis A. The input shaft 64 includes an outer diameter 66 that is less than the inner diameter 74 of the carrier 70. The sun gear 62 includes a pitch diameter 68 that is larger than the inner diameter 74 of the carrier.

The carrier 70 includes an outer diameter 72 that is larger than a pitch diameter 84 of the ring gear 82. Each of the planet gears 102 are supported for rotation within the carrier about a corresponding axis 108. Each axis 108 is parallel to the engine axis A. The example planet gears 102 are hollow and include an inner cavity 104. Each planet gear 102 includes a first end 114 and a second end 116 that are supported by a bearing assembly 90. The bearing assembly 90 includes a rolling element 96 supported between an inner race 92 and an outer race 94. The bearing assembly 90 is in turn supported within the carrier 70.

The carrier 70 rotates about the engine axis A and therefore experiences increased loads during engine operation as compared to gear systems with a fixed carrier. The bearing assemblies 90 therefore experience increased loading. During engine operation the rotating carrier 70, and thereby each of the bearing assemblies 90 encounter gear loads from the planet gear 102 as shown by arrows 110 and also centrifugal load of the rotating gear mass shown schematically as arrow 112. The gear load 110 and centrifugal load 112 are added by vector summation and must be accommodated by the bearing assemblies 90. The load that the bearing assemblies 90 must accommodate is therefore a total load P that is the sum of the centrifugal load indicated by arrow 112 plus the gear tangential loads indicated by arrows 110.

The bearing assemblies 90 include rolling elements 96 that are supported within the carrier 70. The larger the rolling element 96, the more load capacity, and therefore better performance and durability. The example rolling element 96 can be of any rolling element configuration known including roller, ball, tapered roller, spherical as well as other known configurations. Moreover, although the example bearing assembly 90 is shown as a single row of rolling elements 94, multiple rows of rolling elements are also within the contemplation of this disclosure.

Increasing the size of the rolling elements 94 however requires additional space that in turn reduces space for other features of the gear system 48 such as the number of planet gears 102. The number of planet gears 102 may vary between 3 and 5, inclusive. As appreciated, the more planet gears 102, the less load that is incurred on any one planet gear 102 and corresponding bearing assembly 90. Accordingly, the size of the bearing assemblies 90 is balanced against increases in load on each planet gear 102 for the same space defined for the gear system 48. Additionally, more or larger planet gears 102 can increase the overall weight that in turn increases loads on the bearing assemblies 90. The example disclosed gear system 48 therefore includes features enabling efficient configuration and operation within a set defined design space.

Referring to FIGS. 4 and 5, with continued reference to FIGS. 2 and 3, FIGS. 4 and 5 illustrate features of the gear system 48 that are configured to determine the number of planet gears 102 balanced against the size of the bearing assemblies 90 within a set and defined design space. The parameters utilized to configure the size of the bearing assemblies 90 include the outer diameter 72 of the carrier 70. In this example the outer diameter 72 corresponds with the largest outer diameter for each of the forward portion 76 and the aft portion 78. The inner diameter 74 of the carrier 70 is also considered. The outer diameter 72 and the inner diameter 74 define the design space within which the planet gears 102 and bearing assemblies 90 are to operate. A pitch diameter 84 for the ring gear 82 is indicated in FIG. 4 to illustrate that the outer diameter 72 of the carrier 70 may exceed the pitch diameter 84 of the ring gear 82.

The size of the bearing assemblies 90 must fit within the annular space defined between the inner diameter 74 and the outer diameter 72. The bearing assemblies 90 must also provide an operational life that provides the desired durability for the loads encountered during operation. Bearing life is a calculation known to those skilled in the art and provides a measure of the bearing life at given loads. In one example, bearing life L is calculated utilizing the below equation: L=(C/P)^(3.333)

where C is the bearing capacity; and

P is the total load (centrifugal load and gear load).

The bearing capacity C is value determined for the specific rolling element and represents a rated capacity based on material, size, quality level and other mechanical characteristics specific to the rolling element. The bearing capacity C is a value known and understood by one skilled in the art. Larger values for C correspond with larger bearing size and larger load capacity. However as C increases, and thereby the corresponding size of the rolling element 96 increases, the fewer planet gears 102 that can be utilized within the defined space between the outer diameter 72 and the inner diameter 74. Fewer planet gears 102 also result in larger tangential loads required to carry torque generated and input into the gear system 48.

Accordingly, if three (3) planet gears 102 are utilized, the torsional gear load from the sun gear 62 is split three ways. If four (4) planet gears 102 are utilized than the load is split across four (4) planet gears 102. If five (5) planet gears 102 are utilized than the load is split across five (5) planet gears 102.

Referring to FIG. 6, with continued reference to FIGS. 4 and 5, a disclosed process of assembling and configuring the example gear system 48 is schematically shown at 120 and is initiated by first defining the design space. The design space is that space between the inner diameter 74 of the carrier 70 and the outer diameter 72 of the carrier 70. In the disclosed example, the first step is to maximize the carrier outer diameter 72 so it is as large as possible outside the ring gear pitch diameter 84 as is indicated at 122. The carrier inner diameter 74 is than minimized to the smallest diameter possible that will allow the input shaft 64 to extend through the aft portion 78 and still be strong enough to deliver torque to the sun gear 68. The circumference of the average diameter between the outer diameter 72 and the inner diameter 74 is utilized along with the number of planet gears 102 to determine the maximum space remaining for the diameter of the rolling element 96 as is indicated at 126.

Accordingly, the maximum possible size for the rolling element 96 of the bearing assembly 90 is reflected in the below equation.

$\frac{\left( {{{Carrier}\mspace{14mu}{OD}} + {{Carrier}\mspace{14mu}{ID}}} \right)\pi}{\left( {2 \times {number}\mspace{14mu}{of}\mspace{14mu}{planets}} \right)} > {{Diameter}\mspace{14mu}{for}\mspace{14mu}{rolling}\mspace{14mu}{element}}$

The number of planet gears 102 is also dependent on the desired speed reduction ratio. In one disclosed example embodiment the speed reduction ratio is between about 3:1 and about 5:1. This speed reduction ratio range enables a maximum number of planet gears to be no more than five (5) and requires at least three (3) planet gears. Therefore, the step of calculating the possible diameter of the bearing assembly as indicated at 126 includes determining the possible diameter for at least three (3) planet gears 102 and no more than five (5) planet gears 102.

The diameter for the rolling element 96 is therefore made as large as possible between the outer diameter 72 and the inner diameter 74 to fit three (3) planet gears 102 and then reduced in size to fit four (4) and five (5) planet gears 102.

Once the size of the rolling element 96 is determined for each possible number of planet gears 102, life calculations are made for each configuration as indicated at 128. The configuration with bearing life within a desired range of life according to the bearing life calculation is initially selected as the configuration for the gear system 48. Note that “C” utilized in the life calculation is a value that accounts for the diameter of the rolling element bearing and also represents the rated Capacity of the bearing. The Capacity of the bearing is based on size, stresses and material of the selected rolling bearing element. In general, the larger diameter bearing will have a higher capacity “C” for bearings of the same material, construction and quality level.

Many different diameters of the rolling element 96 may provide the desired bearing life expectancy for the loads placed on the gear system 48. However, the designs space, and/or the size of the rolling element is minimized to enable more options in the number of planet gears 102. The design space determination indicated at 126 may be repeated along with the bearing life calculations indicated at 128 to define a smallest bearing space that still provides the desired operational life. Accordingly, the design space is selecting using the largest bearing that will fit between the outer and inner diameters 72 and 74 with the fewest planets. Bearing life is then calculated as indicated at 128. The diameter and/or space for the rolling element is then reduced until an additional planet gear 102 can be utilized to determine if the bearing life still provides an acceptable operational life.

Once a bearing configuration is determined that fits within the defined design space and that meets the desired bearing life requirements, the configuration of the gear system can be finalized by selecting the number of planet gears 102 that fit within the desired life expectancy ranges as indicated at 130.

Accordingly, the disclosed example gear system and configuration method enables assembly and operation that accommodates the increased loading and provides speed reduction within a desired range.

Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For that reason, the following claims should be studied to determine the scope and content of this disclosure. 

What is claimed is:
 1. A gear system for a geared turbofan engine, the gear system comprising: a sun gear driven by a low spool shaft, the sun gear defining a sun gear diameter; a rotating carrier that drives a fan, the carrier defining an outer carrier diameter and an inner carrier diameter; a non-rotating ring gear, the ring gear defining a ring gear diameter, the ring gear diameter is smaller than the outer carrier diameter; and a set of planet gears mounted on corresponding rolling element bearing assemblies, each rolling element bearing assembly is supported within the rotating carrier within a space defined between the carrier outer diameter and the carrier inner diameter such that at least a portion of each rolling element bearing assembly is disposed radially outward of a pitch diameter of the non-rotating ring gear, wherein each of the sun gear, ring gear and planet gears are substantially centered along a gearbox centerline transverse to an engine longitudinal axis and the gear system provides a speed reduction ratio between an input to the sun gear and an output from the carrier between 3:1 and 5:1.
 2. The gear system as recited in claim 1, wherein the set of planet gears comprises between 3 and 5 planet gears, inclusive.
 3. The gear system as recited in claim 1, wherein an outer periphery of each of the rolling element bearing assemblies extends past the ring gear diameter.
 4. The gear system as recited in claim 1, wherein the rotating carrier comprises a forward portion attached to an aft portion with the set of planet gears supported between the forward portion and the aft portion.
 5. The gear system as recited in claim 1, wherein the ring gear diameter comprises a pitch diameter.
 6. The gear system as recited in claim 1, wherein the sun gear diameter is greater than the rotating carrier inner diameter.
 7. The gear system as recited in claim 1, wherein the rolling element bearing assemblies comprise one of a roller, a ball, a tapered roller, or a spherical bearing.
 8. The gear system as recited in claim 1, wherein the rolling element bearing assemblies comprise multiple rows of rolling element bearings.
 9. A method of creating a gear system for a geared turbofan engine, the method comprising: configuring a carrier to include an outer diameter and an inner diameter about a gearbox centerline transverse to an engine longitudinal axis; configuring a sun gear to be supported on an input shaft, wherein the input shaft includes a diameter less than the carrier inner diameter and the sun gear includes a sun gear pitch diameter greater than the carrier inner diameter; configuring a non-rotating ring gear to include a ring gear pitch diameter; configuring a set of planet gears for assembly into the carrier, wherein each of the sun gear, the ring gear and the set of planet gears are configured to be centered along the gearbox centerline; configuring at least one rolling element bearing assembly to include a bearing outer diameter that fits within a space between the carrier outer diameter and the carrier inner diameter; determining a bearing life of the at least one rolling element bearing assembly based upon the bearing outer diameter; and configuring the set of planet gears to include between 3 and 5 gears, inclusive, responsive to the determined size of the bearing outer diameter.
 10. The method as recited in claim 9, including configuring the ring gear pitch diameter to be less than the carrier outer diameter.
 11. The method as recited in claim 9, including selecting the number of planet gears within the set of planet gears to provide the greatest number of planet gears with the determined size of the bearing outer diameter.
 12. The method as recited in claim 9, including configuring the carrier to include a forward portion attached to an aft portion and supporting the set of planet gears between the forward portion and the aft portion.
 13. The method as recited in claim 12, including supporting a rolling element bearing assembly within each of the forward portion and the aft portion.
 14. The method as recited in claim 9, including configuring each of the rolling element bearing assemblies to include at least one of a roller, a ball, a tapered roller, or a spherical bearing.
 15. A geared turbofan engine comprising; a fan section rotatable about an engine longitudinal axis; a turbine section; and a gear system driven by the turbine section, the gear system including: a sun gear driven by a low spool shaft, the sun gear defining a sun gear diameter, a rotating carrier for driving the fan section, the carrier defining an outer carrier diameter and an inner carrier diameter, a non-rotating ring gear, the ring gear defining a ring gear diameter, the ring gear diameter is smaller than the carrier outer diameter, and a set of planet gears mounted on corresponding rolling element bearing assemblies, each of the rolling element bearing assemblies being supported within the carrier within a space defined between the carrier outer diameter and the carrier inner diameter such that a portion of the rolling element bearing assemblies are disposed radially outward of a pitch diameter of the non-rotating ring gear, wherein each of the sun gear, ring gear and planet gears are substantially centered along a gearbox centerline transverse to an engine longitudinal axis and the gear system provides a speed reduction ratio between an input to the sun gear and an output from the carrier between 3:1 and 5:1.
 16. The geared turbofan engine as recited in claim 15, wherein the set of planet gears comprises between 3 and 5 planet gears, inclusive.
 17. The geared turbofan engine as recited in claim 15, wherein an outer periphery of each of the rolling element bearing assemblies extends past the ring gear diameter.
 18. The geared turbofan engine as recited in claim 15, wherein the carrier comprises a forward portion attached to an aft portion with the set of planet gears supported between the forward portion and the aft portion.
 19. The geared turbofan system as recited in claim 15, wherein each of the rolling element bearing assemblies comprise one of a roller, a ball, a tapered roller, or a spherical bearing. 